A black hole 36 billion times heavier than our Sun has been discovered in the Cosmic Horseshoe galaxy. The cosmic colossus is near the upper limit of what astrophysicists believe is possible for black hole size.
The Cosmic Horseshoe is actually a system of 2 galaxies. One galaxy is further away from Earth while the other is on the imaginary line between the more distant galaxy and us.
Cosmic Horseshoe. Credit: NASA/ESA.
So massive is this nearer galaxy that it gravitationally lenses the more distant galaxy. This effect leads to an Einstein Ring – the gravity of the nearer galaxy distorts spacetime, bending the light from the further galaxy so much that it appears like a near-complete ring or horseshoe.
The massive black hole sits in the centre of the nearer galaxy about 5.6 billion light-years from Earth.
Its discovery is detailed in a paper published in the Monthly Notices of the Royal Astronomical Society.
“This is among the top 10 most massive black holes ever discovered, and quite possibly the most massive,” says co-author Thomas Collett from the University of Portsmouth, UK.
Keen black hole enthusiasts are at this point likely mashing their keyboards to confirm that another black hole, which is more than twice the size of the 36-billion-solar-mass giant in the Cosmic Horseshoe, has already been found.
TON 618 is estimated to be 66 billion times bigger than the Sun. The issue is, at about 18.2 billion light-years away, getting a fix on TON’s size is not easy.
“Most of the other black hole mass measurements are indirect and have quite large uncertainties, so we really don’t know for sure which is biggest. However, we’ve got much more certainty about the mass of this [Cosmic Horseshoe] black hole thanks to our new method,” Collett explains.
Astronomers detected the Cosmic Horseshoe black hole with a combination of gravitational lensing and stellar kinematics.
Stellar kinematics is an analysis of the motion – speed, orbit, distance, etc. – of stars around a black hole. Such a study is the best way to determine the size of a black hole but gets harder the further away an object is.
The Cosmic Horseshoe black hole is making stars in the inner galaxy move extremely quickly – about 400km per second! This is 0.1% the speed of light and more than 13 times faster than the speed at which Earth orbits the Sun.
Supermassive black holes are often found at the centre of galaxies. Ultramassive black holes – a term sometimes used for black holes more than 5 billion times bigger than the Sun – are expected to live in the centre of especially hefty galaxies.
Cosmic Horseshoe’s ultramassive black hole is about 10,000 times bigger than Sagittarius A*, the supermassive black hole at the centre of our Milky Way galaxy.
“We think the size of both [galaxies and their central black hole] is intimately linked because when galaxies grow they can funnel matter down onto the central black hole,” Collett says.
“Some of this matter grows the black hole but lots of it shines away in an incredibly bright source called a quasar. These quasars dump huge amounts of energy into their host galaxies, which stops gas clouds condensing into new stars.”
Cosmic Horseshoe with a faint central image close to the black hole which is what made the new discovery possible. Credit: NASA/ESA/Tian Li(University of Portsmouth).
The Cosmic Horseshoe system is no ordinary galaxy. It’s a ‘fossil group’ – the end state of the most massive dense structures in the universe. They emerge when multiple smaller, gravitationally bound galaxies have condensed into one.
“It is likely that all of the supermassive black holes that were originally in the companion galaxies have also now merged to form the ultramassive black hole that we have detected,” Collett explains. “So we’re seeing the end state of galaxy formation and the end state of black hole formation.”
The ultramassive black hole is not active, making its detection an even more impressive feat.
“This discovery was made for a ‘dormant’ black hole – one that isn’t actively accreting material at the time of observation,” says lead researcher Carlos Melo, a PhD candidate at the Universidade Federal do Rio Grande do Sul in Brazil. “Its detection relied purely on its immense gravitational pull and the effect it has on its surroundings.”
“What is particularly exciting is that this method allows us to detect and measure the mass of these hidden ultramassive black holes across the universe, even when they are completely silent,” Melo adds.